Disc drives are common data storage devices. A typical disc drive includes a rigid housing that encloses a variety of disc drive components. The components include one or more discs having data surfaces that are coated with a medium for storage of digital information in a plurality of circular, concentric data tracks. The discs are mounted on a spindle motor that causes the discs to spin and the data surfaces of the discs to pass under respective hydrodynamic or aerodynamic bearing disc head sliders. The sliders carry transducers, which write information to and read information from the data surfaces of the discs.
An actuator mechanism moves the sliders across the data surfaces of the discs. The actuator mechanism includes a motor, such as a voice coil motor (VCM), a track accessing arm and suspensions that support the sliders. Under the control of electronic circuitry, the motor is operated so as to move the actuator mechanism about an actuator pivot center such that the sliders move from track-to-track across the data surface of a disc.
The center of gravity of the actuator is designed to coincide with the rotation axis of the actuator pivot center. However, each part of the actuator, such as the VCM, the track-accessing arm and suspensions, are designed and manufactured separately. Error in manufacturing and assembly of the actuator causes the center of gravity of the actuator to not align with the actuator pivot center. This misalignment is the mass unbalance of the actuator and is determined by multiplying the mass of the actuator by the distance of the actuator center of gravity to the actuator pivot center.
The ability to determine the distance of the actuator center of gravity to the actuator pivot center is crucial for actuator design as well as to ensure disc drive reliability. An actuator having a mass unbalance is especially sensitive to external linear vibrations and shock excitations. In addition, controlling the -mass unbalance in disc drives with high track densities is very important. However, the distance between the center of gravity of the actuator and the actuator pivot center in high track density systems is too low to measure by traditional mechanical methods.